The EU-B-37656 shows a restraining frame for a motor vehicle seat, wherein a support piece is suspended by springs. The support piece consists of two lateral parts connected to the frame by springs and mutually connected by cross elements. There is a centrally located adjusting shaft which can be actuated by a hand wheel, the shaft running through so-called engagement devices, which in turn are connected to the outermost points of the lateral parts via flexible connection elements. When the hand wheel is turned, an arching of the support part is supposed to result, whereby basically in the beginning only an expansion of the springs occurs. This type of construction affords support to only a few vertebrae; the vertebrae lying above and below do not get the same strong support. A freely suspended insertion of the support part in the backrest or a construction wherein the support is suspended in the upholstery is not possible.
The DE-OS 2804703 shows a backrest wherein a plate having straps arranged oppositely to each other can be arched via mutually engaged guide rails. The entire plate is height-adjustable by means of a rocker lever.
The EU-A1-322535 shows a backrest with an upper and a lower support strap which are interconnected by a vertical, flexible strap and a spring tensioned between them, whereby an adjusting shaft engages via a gear in the lower support strap, in order to achieve an arching of the backrest by bending the flexible strap.
Further, backrests made of foam material are known, which are supposed to adjust to the shape of the spinal column.
Another example is shown by DE-OS 3440846 which discloses an automobile seat whose backrest comprises a frame and a plate received in the frame. The plate is provided with vertically extending longitudinal slits in which a lumbar-support device is movable. The lumbar support can be adjusted by a hand wheel through a shaft linked onto the frame to vary its stiffness and to move it vertically.
The lumbar support is directly connected with the frame. As a result, all vibrations imparted by the road to the vehicle are fully transmitted to the back of the driver.
The known mechanisms do not permit any accommodation to changed or changing curvatures of the spinal column. For instance, during long car trips, the curvature of the spinal column alters greatly as a result of the required holding work, when the total weight of the upper body must be supported based on this disadvantageous position (hand stretched out towards the steering wheel). This muscle work, respectively holding work in order to keep the body upright under conditions of improper or insufficient support has to be performed entirely by the musculature and the skeletal system, and thus logically leads to considerable exhaustion, as well as muscle stress. The involved muscles require for this work up to 40 times more blood, respectively 40 times more energy, compared to an anatomically supported upper body (whose muscles do not have to perform any holding work).
The support, respectively holding work must be performed by the arching mechanism and namely for all spinal columns (whose curvatures are as varied as human faces—and which in addition, are constantly altered from hour to hour due to tiredness, exertion and the required holding work—the height of a person decreases from morning to evening be approximately 2 to 4 cm, and the spinal curvature changes in this time period by up to 10 cm). Thus, an optimal arching mechanism has to act in a differentiated manner so that for instance when used in a backrest, it can suitably support any shape of back, i.e. at least threefold and with variable strength, as follows: 1. very strongly and stably in forward direction in the pelvic area (against the backward tilting of the pelvis), 2. somewhat gentler but equally firm radially and vertically in order to support each vertebra in the lower-back (lordosis) area and 3. essentially stronger and vertically in an upward direction in the pectoral area under the shoulders against the weight of the upper body. The additional support for the neck area can be a further requirement.
Once the arch is set and adjusted to the shape of the spinal column, it is naturally not allowed to change its shape even under fully exerted pressure, in order to insure full support independently from the momentary curvature of the spinal column of the user.
The at least triple support required from an ergonomically optimal backrest can not be offered at all or only insufficiently by the known devices.